Re: [PATCH v5 6/9] ALSA: virtio: PCM substream operators

[Takashi Iwai <tiwai@suse.de>]

On Thu, 25 Feb 2021 13:14:37 +0100,
Anton Yakovlev wrote:
> 
> On 25.02.2021 11:55, Takashi Iwai wrote:
> > On Mon, 22 Feb 2021 16:34:41 +0100,
> > Anton Yakovlev wrote:
> >> +static int virtsnd_pcm_open(struct snd_pcm_substream *substream)
> >> +{
> >> +     struct virtio_pcm *vpcm = snd_pcm_substream_chip(substream);
> >> +     struct virtio_pcm_substream *vss = NULL;
> >> +
> >> +     if (vpcm) {
> >> +             switch (substream->stream) {
> >> +             case SNDRV_PCM_STREAM_PLAYBACK:
> >> +             case SNDRV_PCM_STREAM_CAPTURE: {
> >
> > The switch() here looks superfluous.  The substream->stream must be a
> > good value in the callback.  If any, you can put WARN_ON() there, but
> > I don't think it worth.
> 
> At least it doesn't do any harm.

It does -- it makes the readability worse, and that's a very important
point.

> >> +static int virtsnd_pcm_hw_params(struct snd_pcm_substream *substream,
> >> +                              struct snd_pcm_hw_params *hw_params)
> >> +{
> > ....
> >> +     return virtsnd_pcm_msg_alloc(vss, periods, period_bytes);
> >
> > We have the allocation, but...
> >
> >> +static int virtsnd_pcm_hw_free(struct snd_pcm_substream *substream)
> >> +{
> >> +     return 0;
> >
> > ... no release at hw_free()?
> > I know that the free is present in the allocator, but it's only for
> > re-allocation case, I suppose.
> 
> When the substream stops, sync_ptr waits until the device has completed
> all pending messages. This wait can be interrupted either by a signal or
> due to a timeout. In this case, the device can still access messages
> even after calling hw_free(). It can also issue an interrupt, and the
> interrupt handler will also try to access message structures. Therefore,
> freeing of already allocated messages occurs either in hw_params() or in
> dev->release(), since there it is 100% safe.

OK, then it's worth to document it about this object lifecycle.
The buffer management of this driver is fairly unique, so otherwise it
confuses readers.


thanks,

Takashi